1. Field of the Invention
The present invention relates to a high-frequency circuit device including two parallel planar conductors, such as a waveguide or a resonator, and to a transmitter/receiver including the same.
2. Description of the Related Art
In general, as high-frequency circuit devices using high-frequency signals, such as microwaves and millimeter waves, the following various transmission lines have been known: a grounded coplanar transmission line, in which a ground electrode is formed on the bottom surface of a dielectric plate and a coplanar is formed on the top surface thereof; a grounded slot transmission line, in which a ground electrode is formed on the bottom surface of a dielectric plate and a slot is formed on the top surface thereof; and a Planar Dielectric Transmission Line (hereinafter referred to as a PDTL), in which slots are formed on both principal surfaces of a dielectric plate such that the slots face each other.
Each of these transmission lines includes two planar conductors which are parallel to each other. Thus, if an electromagnetic field is disturbed at the input/output unit or a bend of the transmission line, an undesired wave of a spurious mode, such as a so-called parallel plate mode, may be generated between the two parallel planar conductors. Accordingly, the undesired wave propagates between the planar conductors and interference of undesired wave may be caused between adjoining transmission lines, whereby leakage of a signal may be disadvantageously caused.
In order to prevent propagation of such an undesired wave, a spurious-mode propagation preventing circuit (for example, disclosed in Japanese Unexamined Patent Application Publication No. 2000-101301) has been known. In this circuit, an electrode is provided on a planar conductor formed on the top surface of a dielectric plate, the electrode generating capacitance between planar conductors on the top and bottom surfaces. Also, a conductive pattern which is connected to the electrode so as to form an inductor and which includes a plurality of lines is provided.
In the above-described known art, the conductive pattern including the electrode and the lines is formed on the planar conductor on the top surface of the dielectric plate and the capacitance of the electrode and the inductance of the lines are combined so as to form a low-pass filter so that propagation of an undesired wave can be prevented. However, in the known art, the capacitance of the electrode or the inductance of the lines must be increased as the frequency of the undesired wave decreases.
At this time, the electrode generates a capacitance between the electrode and the planar conductor on the bottom surface. Thus, the area of the electrode must be increased in order to increase the capacitance. On the other hand, in order to increase the inductance, the width of each line should be decreased or the length of each line should be increased. Since the width of the line is limited by the manufacturing precision, the length of the line must be increased in order to increase the inductance.
Accordingly, in the known art, the area of the conductive pattern is likely to increase, and thus the size of the entire dielectric plate increases and the manufacturing cost also increases.